A type of silicon-on-insulator (SOI) semiconductor device comprises a substrate of an insulating material, such as sapphire or silicon oxide, having islands of single crystalline silicon thereon. Various semiconductor devices, such as transistors, are formed in the islands and are electrically connected to form a desired circuit. The islands are formed by forming a layer of single crystalline silicon on the insulating substrate, covering the portion of the silicon layer which are to form the islands with a masking material, and then removing the portions of the silicon layer not covered by the masking layer. The uncovered portions of the silicon layer can be removed by various techniques, such as by a wet chemical etch or by a plasma etch.
FIG. 1 shows a silicon island 10 on an insulating substrate 12 of sapphire which island 10 was formed by a wet chemical etch. This etching technique leaves the island 10 with sharp corners 14 between the tapered sides walls 16 and the top surface 18. FIG. 2 shows a silicon island 20 on a sapphire substrate 22 formed by a plasma etch. This technique also provides the island 20 with sharp corners 24 between the straight vertical side walls 26 and the top surface 28. The sharp corners provide regions of very high electric field which results in a significant limitation to the dielectric integrity of the channel oxide of a MOS transistor formed in the island. In order to delete this high field region it would be desirable to remove the sharp corners and form the islands with rounded corners. Past approaches to the creation of such a rounded corner are described in the article "Dielectric Integrity of the Gate Oxide in SOS Devices", by R. K. Smeltzer et al published in RCA REVIEW, Vol. 45, June, 1984, pages 194-229. However, rounding the corners by means of a wet chemical etching operation provides a very rough surface so that no significant improvement in electrical properties was achieved. Likewise plasma etching provides corners having a rough surface. Therefore, it would be desirable to have a method of forming the silicon islands with rounded corners which are smooth.